The modern gas turbine is an energy producing system based on the Brayton Cycle. The basic unit consists of a compressor which takes ambient air and increases its pressure and temperature, a combustion chamber where the pressurized gas temperature is increased to a higher level through the combustion of fuel, and an expansion turbine where the hot pressurized gas is expanded and cooled resulting in the output of work. Some of the work performed by the expansion turbine is used to drive the compressor and the remaining work is used to turn a generator, turn a reduction gear, compress a gas, or in the case of a jet aircraft engine becomes the thrust used for propulsion.
Over time, the gas turbine has been improved, and the single shaft turbine has evolved into a two-shaft version where the compressor combustion chamber and high pressure turbine to drive the compressor are on one shaft, while the power output turbine is on a separate shaft which in turn drives the generator, gear box, or in the case of aircraft engines, the high ratio bypass fan. Since a gas turbine is a constant combustion engine, the heat resistance of the materials that make up the engine components has limited the thermodynamic cycle temperatures. With modern materials and localized cooling, modern gas turbines have been able to achieve combustor outlet temperatures approaching 2500° F. However, the localized cooling also bleeds off some of the compressed air and in turn can reduce the engine efficiency.
Other attempts have been made to improve the Brayton Cycle efficiency by adding additional steps to the cycle. One of these steps includes interstage cooling of the air in the compressor. In this process, air is removed from the compressor and passed through a heat exchanger where some of the heat of compression is removed. Then the air is reinjected into the next compressor stage for further compression. Interstage cooling can be performed on one or more stages of a single compressor or between two compressors where they are connected in series.
Another addition to the Brayton Cycle that is used to improve the cycle efficiency is regeneration. Regeneration requires the installation of a heat exchanger that takes the compressed air from the compressor outlet and increases its temperature prior to the compressed air entry into the combustor. The heat to increase the compressed air temperature comes from the exhaust gas from the last turbine in the engine. The energy used to heat the compressed air is recovered from the exhaust stream and increases the overall system efficiency. It also improves the efficiency of the gas turbine when it is operating at less than full power. Regenerators are usually used on low pressure compressors because high pressure compressors have such high outlet air temperatures that there is not enough differential temperature between the compressor outlet and the turbine exhaust stream to make heat recovery practical.
The Brayton Cycle performance can also be improved by adding reheat to the pressurized hot air as it passes through the various expansion phases of the compressor drive turbine, the power output turbine, or in the special case of jet aircraft engines, the afterburner where the additional energy is used to increase jet thrust. The process of adding reheat is far more complex than just adding additional fuel, because as additional fuel is added, the oxygen content of the pressurized hot air is diminished until the flammability limits are reached and the air will no longer support the combustion process. In many cases, the prior art describes reheat processes which cannot be achieved either because there is insufficient oxygen in the pressurized air or even if the remaining oxygen were to be consumed, the flame temperature would never be high enough to meet the performance claims of the engine.
An analysis of the prior art finds eighteen U.S. patents which discuss reheating of the process gas between expansion turbines. These patents can be grouped into three classes which describe their primary teachings. The first class is designed to use process gas reheat in a gas turbine that has a regeneration system in it. These turbines are usually small in capacity and are outside the scope of this invention. U.S. Pat. No. 2,242,767 to Traupel shows a regeneration unit used on a gas turbine with interstage reheat on the compressor drive turbine, U.S. Pat. No. 2,407,166 to Kreitner et al. shows a regeneration unit with a compressor with interstage cooling and a turbine with interstage reheat, U.S. Pat. No. 2,755,621 to Terrell in FIG. 3 shows a hot gas valving system for pressure control in a turbine with reheat and a regeneration system, and U.S. Pat. No. 3,054,257 to Schelp shows a small gas turbine for vehicles which has process gas reheat and two regeneration units in a single gas turbine unit. None of these units use superheat to bring the power turbine into adiabatic isentropic balance to optimize the power output.
The second class of patents which teach of the advantages of process gas reheat are the single shaft turbine units. In each of these patents, there are one or more shafts that contain a compressor unit, a combustor unit, and an expansion turbine on the same shaft. In each of these gas turbines, the efficiency gains of using superheat described in this invention are lost on a turbine which is mounted on the same shaft as the compressor unit. Patents which belong to this class include: U.S. Pat. No. 1,988,456 to Lysholm shows a single compressor with a water cooled combustor feeding hot process gas to two of four expansion turbines that have reheat combustors and are mounted on a single shaft. U.S. Pat. No. 2,504,181 to Constant shows an aircraft turbine with two shafts mounted end to end and each shaft has a compressor, a combustor, and a turbine. U.S. Pat. No. 3,765,170 to Nakamura shows a dual shaft gas turbine with each shaft having a different capacity and consisting of a compressor, a regenerator, a combustor, and a turbine. U.S. Pat. No. 3,867,813 to Leibach shows in FIG. 2 an aviation engine with three concentric shafts each with a compressor and a turbine mounted thereon with a single combustor to normally drive two of the shafts and an intermittent reheat combustor to drive the low pressure compressor and turbine unit. U.S. Pat. No. 5,313,782 to Frutschi et al. shows a gas turbine in combination with a steam power unit where the gas turbine consists of either two compressors with intercooling, a combustor, a power turbine, a reheat combustor, and a second turbine on a single shaft, or the same compressors and turbines mounted on two concentric shafts respectively. U.S. Pat. No. 5,347,806 to Nakhamkin shows a gas turbine power plant with two or more compressors, intercoolers, combustors, and turbines all cascaded together with a regenerator to improve efficiency. U.S. Pat. No. 5,454,220 to Althaus et al. shows a single shaft turbine unit which has a compressor, a primary combustion chamber, a turbine, an auto-ignition reheat combustion section, and a second turbine. U.S. Pat. No. 5,465,569 to Althaus et al. shows a single shaft turbine unit which has a compressor, a primary combustion chamber, a turbine, a reheat combustion chamber, and a second turbine where the fuel flow to the reheat combustion chamber is used to modulate higher end power settings. U.S. Pat. No. 5,577,378 to Althaus et al. shows a single shaft turbine unit which has a compressor, a primary combustion chamber, a gas turbine, an auto-ignition reheat combustion section, a second gas turbine, and a steam turbine. U.S. Pat. No. 6,079,197 to Attia shows a gas turbine system consisting of a low temperature compressor, a combustor, and a high pressure turbine on a single shaft plus a high temperature compressor, a reheat combustor, and a low pressure turbine on a single shaft or the two turbine units all mounted on one shaft. In each case, the patents listed above show the reheat energy applied to a turbine which is mounted on the same shaft with a compressor which does not apply to the new invention.
The third class of patents which teach the advantages of process gas reheat are the special units. These patents are for unique processes which are different from those included in this invention. U.S. Pat. No. 4,206,593 to Su et al. shows a gas turbine with a compressor, a combustion chamber, a first turbine, a reheat combustor of a venturi design, and a second turbine all mounted on a single shaft. U.S. Pat. No. 4,885,912 to Nakhamkin shows a power system where air is compressed, stored and then released to a recuperator where it is heated and expanded in a high pressure turbine without combustion. Then the air is mixed with fuel and combusted where it is expanded in a low pressure turbine and exhausted to the recuperator. U.S. Pat. No. 5,133,180 to Horner et al. shows a gas turbine consisting of a compressor, a combustion chamber, and a compressor drive turbine on a shaft. A second shaft has a power output turbine. A reformer is integrated into the system to provide fuel to the primary combustor. The power output turbine provides hot gas to the reformer and does not have reheat (col. 3, lines 12-17). U.S. Pat. No. 5,184,460 to Franciscus et al. shows a two-shaft aviation turbine with a compressor, a first combustor, a first turbine, a second combustor, a second turbine on one shaft and a fan, a third combustor, a third turbine, a fourth combustor, and a fourth turbine on the second shaft. All the combustors operate at the same temperature of 2800° R.
All the patents in the special class can be distinguished from this invention because they do not address the complex relationship between superheat temperature, process gas pressure, flammability limits, and material temperature limits. All of the variables are brought together in this invention to develop the proper superheat levels to obtain the greatest power output possible at minimum fuel consumption levels.